Introduce proper backend config merge

[lakkeger]

Motivation

Backend configs are currently merged selectively, due to this some options are not usable as the override file's takes precedent over the user defined backend options.

Note: users have to use the right use_path_style/force_path_style option for the given provider, this is not handled by tflocal.

Changes

• introduces backend configuration merging, on top of the default backend options new ones only added to the dict if the user defines them
• tests backend merging by adding test_s3_backend_configs_merge, checks generated override file for the following extra options added by the "user":
  • acl
  • encryption
  • use_path_style

[cloutierMat]

Awesome!

Thanks for adding this. It will help greatly with allowing a more configurable backend for our Terraform configuration.

I just left a nit, feel free to ignore if you think it helps with code readability.